Ice Contaminated Tailplane Stall
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The term "Ice Contaminated Tailplane Stall", or ICTS, refers to those events that involve flow separation from the horizontal stabilizer, due to ice accretion, which leads to an aerodynamic stall of the tailplane and result in a nose-down pitch upset of the aircraft.
Some aeroplane types are prone to a nose-down pitch upset, referred to as a "tailplane stall", due to ice contamination of the horizontal stabiliser. It has been determined that aircraft with reversible, unpowered, elevator control surfaces are more prone to tailplane stall than those with other control configurations.
In most aircraft, the centre of gravity (cg) is somewhat forward of the centre of pressure. The exact distance between the cg and the centre of pressure will depend on aircraft loading, configuration, thrust setting and drag. However, the fact that it is forward of the centre of pressure results in a nose-down pitching moment. The purpose of the horizontal stabilizer, or tailplane, is to provide a downward force to overcome this normal, nose-down, pitching tendancy. If the horizontal stabiliser becomes contaminated with ice, airflow separation from the surface can prevent it from providing sufficient negative lift or downward force to balance the aircraft and a nose-down pitch upset can occur.
When compared to the wing, the horizontal stabiliser, of a given aircraft, normally has a relatively thin aerofoil profile. Due to differences in the ice collection efficiency or catch rate between the two surfaces, ice can accumulate on the horizontal stabiliser before any ice is present on the wing.
A tailplane stall can occur at relatively high speeds, well above the normal 1G stall speed of the wing. Typically, the stall will occur near the flap limit speed as it is most likely to take place when the flaps are extended to the landing position; especially when extension is combined with a nose down pitching manoeuvre, airspeed change, power change or flight through turbulence. As aircraft stall warning systems provide warnings based on the stall speed of an uncontaminated wing, during a tailplane stall induced upset, there will be NO artificial stall warning indications, such as a stick shaker, warning horn or the buffeting normally associated with a wing stall.
Tailplane Stall Aerodynamics (Simplified)
- The horizontal stabiliser, or tailplane, of an aircraft is an aerofoil which provides a downward force to overcome the aeroplane's normal nose-down pitching moment. The further forward the centre of gravity is from the center of pressure, the greater the nose down moment and, thus, the greater the amount of force that must be generated by the tailplane. This, in turn, requires a greater tailplane angle of attack (AOA).
- Accumulation of ice on the tailplane will result in disruption of the normal airflow around that surface and will reduce the critical (or stalling) AOA of the horizontal stabiliser.
- Ice can accumulate on the tailplane before it begins to accumulate on the wing or other parts of the aircraft.
- Extension of the flaps usually results in an aft movement of the centre of pressure. This lengthens the arm between the centre of pressure and the centre of gravity resulting in an increase in the nose down moment of the aircraft. More force is required from the tailplane to counter this moment necessitating a higher AOA.
- Extension of flaps, especially near the maximum extension speed, and/or increasing the power setting on a propeller driven aircraft can increase the AOA of the tailplane due to an increase in downwash.
- A tailplane stall induced upset will occur when the critical AOA of the horizontal stabiliser is exceeded causing it to stall.
- Stall of the tailplane drastically reduces the downward force it produces resulting in a rapid aircraft nose-down pitching moment.
- The nose-down pitching moment can trigger an aerodynamic overbalance of an un-boosted elevator which, in turn, will result in an abrupt nose-down deflection of the elevator surface(s). The control force required for the pilot to return the elevator to neutral or to a nose-up deflection can be significant.
Tailplane Stall Indications
Indications of an impending tailplane stall, as determined under test flight conditions, include:
- Sudden changes in elevator force with a tendency for the control column to move forward if not restrained
- Elevator control oscillation or pulsing with forward movement of the yoke much lighter than a corresponding aft movement. This can often lead to pilot induced oscillation (PIO)
- Reduction in elevator effectiveness
- A forward control column movement or an abrupt nose down pitch change with a change in power setting
- NO indications of an approaching stall such as airframe buffet or activation of stall warning systems
Note that these indications might not become apparent until after the flaps have been partially extended.
Tailplane Stall Prevention
Note that in ALL cases, manufacturer's procedures and Aircraft Flight Manual (AFM) guidance take precedence over any recommendations of this article
If, in an aircraft type susceptible to tailplane stall, an approach in icing conditions cannot be avoided, the pilot should:
- Plan to fly the approach at the minimum allowable flap setting for the conditions.
- Avoid abrupt pitching manoeuvres and power changes. Nose down pitching motion and power changes in propeller driven aeroplanes can increase the tailplane AOA.
- Fly the approach "on speed" for the configuration. Avoid extension of the flaps near the flap limit speeds.
- If indications of an impending stall occur shortly after flap extension, immediately retract the flaps to the previous setting and increase the airspeed to the minimum manoeuvring speed for the reduced flap setting.
- Make any nose down pitch changes gradually, even in turbulent conditions.
Tailplane Stall Recovery
Note that manufacturer's procedures and Aircraft Flight Manual (AFM) guidance take precedence over the following recommendations
Note also that the procedures for recovery from a tailplane stall are essentially the opposite of those required to recover from a wing stall. Should the situation be mis-diagnosed and the wrong procedures applied, a critical situation can be made significantly worse.
If a sudden, un-commanded nose-down pitch occurs in combination with the symptoms of an impending tailplane stall, consider the following recovery actions:
- Disengage the autopilot
- Manually resist any nose down elevator movement
- Immediately retract flaps to previous position if the pitching moment occurred during flap extension
- Return power to the previous setting if the pitching moment occurred during a significant change in power setting. Note that in some aircraft types, an immediate power reduction may be appropriate as part of the initial recovery actions.
- "Tailplane Icing - too much data, not enough knowledge" - an article published in the September 2009 edition of Professional Pilot magazine, by Captain Donald Van Dyke.
National Aeronautics and Space Administration (NASA)